Various types of electrically addressable memory devices for computer data storage are known in the art. Most of these devices store a data bit as a charge in a capacitor. The charge state can be read out and the output signal used to control processes in a computer processor. Most of these devices require complex silicon processing steps and a dedicated device architecture which depends on memory type.
Memory devices are distinguished by their speed and data retention characteristic. Dynamic random access memory (DRAM) is a volatile memory characterized by a destructive read. This means that it is necessary to supply voltage to the memory bits at all times, or the information will disappear. Furthermore, each memory element has associated with it a transistor. Static random access memory (SRAM) stores data in a bistable flip-flop, commonly consisting of cross-coupled inverters. It is called “static” because it will retain a value as long as power is supplied. It is still volatile, i.e. it will lose its contents when the power is switched off, in contrast to ROM. SRAM is usually faster than DRAM, but each bit requires several transistors (about six), so that a lesser number of bits of SRAM fit in the same area as compared to DRAM.
Erasable programmable read only memory (EPROM) is a type of storage device in which the data is determined by electrical charge stored in an isolated (“floating”) MOS transistor gate. The isolation is good enough to retain the charge almost indefinitely (more than ten years) without an external power supply. The EPROM is programmed by “injecting” charge into the floating gate, using a technique based on the tunnel effect. This requires higher voltage than in normal operation (usually 12V–25V). The floating gate can be discharged through UV-illumination or electrically (EEPROM). Usually bytes or words can be erased and reprogrammed individually during system operation. EEPROM is more expensive and less dense than RAM. It is appropriate for storing small amounts of data which is changed infrequently. Another known non-volatile memory device is a ferromagnetic RAM (Fe-RAM), wherein the individual storage cells do not require a dedicated transistor.
When using passive memory devices connected in parallel, e.g., in form of an array, leakage current problems can occur. Leakage problems can be reduced, for example, by forming a Schottky barrier between an organometallic charge-transfer complex, such as M(TCNQ) and the underlying electrode. Alternatively, it has been proposed to provide a rectifying diode in series with the switching resistance at each intersection point of a memory array. This requires additional layers and processing steps.
The molecular composite material can have stable and metastable states that affect the retention time of the device. The device has an electrically insulating off state and at least one electrically conducting on state. The device can be switched reproducibly between the on state(s) and the off state by applying an electrical field across the device with a predetermined polarity and magnitude for a predetermined time.
It would therefore be desirable to provide a device that has a built-in barrier that reduces leakage currents.